Phosphor-converted LEDs (pcLEDs) are currently employed in a wide range of applications, such as for white light LEDs, amber LEDs, etc. Phosphor-converted LEDs comprise an active primary LED light source (typically a III-nitride p-n junction emitting blue light) and a passive secondary light source (phosphor) that absorbs part of the primary light and down-converts it to a secondary light of lower energy. The combination of the blue light leaking through the phosphor layer and the secondary light can produce a wide range of colors. Multiple phosphors may be used to contribute different wavelengths. The secondary light source is not necessarily a phosphor but may be, for example, a quantum dot layer, so we can more generally describe such LEDs as down-converted LEDs (dcLEDs).
The first white LEDs comprised a blue primary light source (an InGaN/GaN junction LED die) having a green emitting phosphor coating (Y3Al5O12:Ce (YAG)). The Ce activator of the YAG phosphor absorbs a part of the blue primary light and emits a broad emission centered in the green. The resulting emission spectrum of the LED is therefore the combination of blue and green light, appearing white. State of the art LEDs now employ a combination of multiple phosphors with a wide range of emission possibilities. Typical warm white LEDs contain at least one green and one red emitting phosphor.
The phosphor layer may be formed in a number of ways, such as mixing phosphor powder in a transparent binder (e.g., silicone, glass, epoxy) and depositing the mixture on top of the blue LED die, or attaching a pre-formed phosphor tile to the LED die with a transparent adhesive (e.g., silicone), or depositing the phosphor over the LED die using electrophoresis. A pre-formed phosphor tile is typically made by sintering phosphor powder under pressure.
It is also known to embed phosphor into a solid transparent matrix (e.g., glass) to create a luminescent substrate, then deposit a seed layer over the substrate, and then epitaxially grow the LED layers over the seed layer.
Some drawbacks of the above-mentioned pcLEDs include the following.
The phosphor layer covering the blue LED die induces scattering of the primary light and thereby reduces the conversion efficiency.
The transparent binders for creating a phosphor mixture, and the adhesives used for attaching a phosphor tile to the LED die, have major disadvantages, such as having an index of refraction lower than the III-nitride and the phosphors, which reduces the conversion efficiency, and their thermal conductivity is low, which reduces LED efficacy and reliability.
Substrates infused with phosphor are inadequate substrates for epitaxial growth of the III-nitride junction due to lattice mismatches and different coefficients of thermal expansion (CTE). Growing on these substrates requires the deposition of a seed layer, which can reduce the junction quality and performance, and is expensive. The CTE mismatches still remain.
The above-mentioned secondary light sources' intensities and wavelengths cannot be tuned once integrated into the LEDs. This can lead to a large spread in color within the produced LEDs. LEDs not meeting the target color criteria result in a lower production yield and an increase of the overall LED cost.